1. Field of the Invention
The present invention relates to an inverter type resistance welding machine having one rectifer circuit and one inverter connected to the rectifier and welding control elements connected between an output of the inverter and a plurality of resistance welding machines.
2. Prior Art
A typical prior art arrangement of a resistance welding machine will be described with reference to FIGS. 4 to 6.
In a first aspect of the prior art arrangement, there are provided a plurality of rectifiers 34, 35, 36, a plurality of inverters 37, 38, 39 respectively connected to the plurality of rectifiers 34, 35, 36, a plurality of transformers 40, 41, 42 respectively connected to the inverters 37, 38, 39, and a plurality of welding machines 31, 32, 33 respectively connected to each secondary winding of each welding transformer 40, 41, 42 respectively independent of each other.
In a second aspect of the prior art arrangement, there are provided a rectifier 51, an inverter 52 connected to the rectifier 51, a tranformer 53 connected to the inverter 52, and a plurality of welding machines 54, 55, 56 each connected to a secondary winding of the welding transformer 53 and arranged in parallel with each other as illustrated in FIG. 5.
In a third aspect of the prior art arrangement, there are provided a rectifier 61, an inverter 62 connected to the rectifier 61, a plurality of transformers 66, 67, 68 each connected to the inverter 62, and a plurality of welding machines 63, 64, 65 connected to a secondary winding of each transformer and arranged in parallel with each other as illustrated in FIG. 6.
There are following problems in the first to third aspects of the prior arts.
In the first aspect of the prior arts, inasmuch as there are provided rectifiers 34, 35, 36 and inverters 37, 38, 39 respectively connected to the rectifiers 34, 35, 36 for each welding machine 31, 32, 33, constant current control thereof can be made independently. However, such constant current control for each welding machine involves very high cost and large space for installing thereof.
Furthermore, inasmuch as the welding current is supplied freely to each welding machine 31, 32, 33, there is a case that a timing for applying or supplying the welding current to a plurality of welding machines is overlapped at some welding machines, which entails a large voltage drop of a power supply 43 to deteriorate welding quality and generate troubles in a welding current control unit. To prevent these problems, a large scale of power supply installment is required.
Still furthermore, in the second and third aspects of the prior arts, it is necessary to apply or supply pressure and welding current to each welding machine considering the constant current or the space for installing the current control unit. In this case, no pressure is applied to other welding machines which are each at a standby condition just after the pressure, the welding current is applied to and kept applied to one welding machine. Hence, it was necessary to start welding operation for the other welding machines just after completion of welding operation of one welding machine, which entails much time.